1. Field of the Invention
The present invention relates to polyurethane-polyacrylate hybrid dispersions which contain allophanate groups and to their use for wood and furniture coatings which exhibit good resistance to staining liquids.
2. Description of the Prior Art
With the aim of reducing emissions of organic solvents, aqueous coating compositions are increasingly being used instead of solvent-containing compositions. Polyurethane dispersions and polyacrylate dispersions are important classes of aqueous lacquer binder vehicles. The various options for the production of polyurethane dispersions have been compared, for example, in a review article by D. Dieterich (D. Dieterich, Prog. Org. Coatings 9,281(1981)). Polyurethane dispersions and poly-acrylate dispersions are often mixed in order obtain high-quality surface properties.
Furthermore, it is also known, particularly from the patent literature, that the polymerization by a radical mechanism of suitable vinyl monomers can be conducted in the presence of aqueous polyurethane dispersions. Examples of patent applications relating to the production of polyurethane-polyacrylate hybrid dispersions such as these include DE-A-1,953,348 and EP-A 167,188, EP-A 189,945 and EP-A308,115. When compared to a mixture of polyurethane and polyacrylate dispersions, the hybrid dispersions are of higher quality.
A considerable improvement of the performance of aqueous coatings has been achieved by the use of modern technologies. Thus, for example, EP 358 979 describes how the resistance of coatings prepared from aqueous acrylate dispersions can be improved by the addition of polyisocyanates. It is also possible to improve the coating properties of polyurethane dispersions by the addition of polyisocyanate crosslinking agents which have been rendered hydrophilic (e.g. EP-A 469,389).
In addition to the development of two-component systems of this type, attempts have been made to also achieve a significant improvement in coating properties for one-component systems, which can be applied more easily. Thus EP-A 332,326 describes how polyurethane-polyacrylate hybrid dispersions can be crosslinked by the condensation reaction of hydrazide and carbonyl groups. The oxidative crosslinking of polyurethane and polyurethane-polyacrylate hybrid dispersions is also known (Advances in Urethane Science and Technology, Volume 10, pages 121-162 (1987), 4th Nurnberg Congress "Creative Advances in Coatings Technology", Paper 17, (1997)).
The properties of lacquers can be somewhat improved by the process steps described above, but the chemical resistance of lacquer films of polyurethane-polyacrylate hybrid dispersions, particularly their resistance to staining liquids, is unsatisfactory.
An object of the present invention is to improve the properties of aqueous, one-component polyurethane-polyacrylate hybrid dispersions, and in particular their resistance to staining liquids.
This object may be achieved with the polyurethane-polyacrylate hybrid dispersions according to the invention, which are based on special polyurethane dispersions containing allophanate groups and exhibit a decisive improvement in resistance to staining liquids.